The UHA-4 is the first Leckerton amplifier to incorporate headphone crossfeed. Some love it and just leave it always enabled, others prefer the sound without it and may never use it. I tend to use it sparingly, only on recordings with hard-panned instruments. In any case, the topic deserves some additional insight. First of all, what is crossfeed? When we listen to a recording with headphones or earphones, the right ear gets only the right channel, and the left ear gets only the left channel. If a sound occurs only in the left channel, it's heard only in the left ear. The right ear gets nothing. This almost never occurs naturally due to a principle called diffraction. Diffraction is essentially the ability of a sound wave to bend around an obstacle. For example, when you are sitting in front of a stereo pair of loudspeakers, there may be no direct path from the left speaker to your right ear (because your head is in the way), but some of the sound from the left speaker is able to bend around your head and reach your right ear. The lower the frequency of the sound (compared to your head), the better it is able to diffract around your head. Really high frequency sounds mostly get absorbed by your head, but the low-frequency stuff just goes right around as if your head weren't there. As the sound is bending around your head to get to the opposite ear, it has to travel a longer distance, so there is also a slight delay of the sound to that opposite ear. Headphone crossfeed aims to simulate these three effects: diffraction, head absorption, and time delay. This is why it is sometimes referred to as an acoustic simulator. It simulates the acoustics of loudspeaker listening.
The idea of a simple, passive crossfeed circuit has been around for quite a long time. Siegfried Linkwitz described a circuit back in 1971. In fact, the "Linkwitz" crossfeed circuit is the basis for many crossfeed designs still in use today, including the UHA-4. The version used in the UHA-4 has a modified configuration which allows true bypass when the switch is off. The circuit is isolated between two active stages, which means it cannot be affected by the source impedance or the load impedance. In other words, its sound doesn't change with different headphones or sound sources.
If you'd like to read more about crossfeed in general, I've seen plenty of great explanations and illustrations out there, including this one: HeadRoom Learning Center: Headphone Imaging
Why use crossfeed? Quite simply, it can make headphone listening more natural and less fatiguing, especially over long periods of time. If you are sometimes bothered by that "inside-your-head" sound which headphones can give, crossfeed can reduce that.
How does the UHA-4 crossfeed sound? It is designed to have minimal impact on overall frequency response. The effect depends on the nature of the recording. With a very wide sound stage or hard-panned instruments, the crossfeed moves the image forwards and away from your ears. On a recording which has a narrower soundstage to begin with, the effect is more subtle. Like I mentioned, I don't use it often, but I'm glad I have it available when I need it.